1. Field of the Invention
The present invention relates to a waveform equalizer that is used in digital wireless communication such as digital broadcasting, for reducing channel distortion of a digital signal.
2. Related Art
Initially, digital broadcasting has predominantly been carried out in satellite broadcasting. In recent years, however, a tide of digitization is spreading to terrestrial broadcasting too. Waveform equalization for removing channel distortion of a digital signal is indispensable in digital terrestrial broadcasting. A conventional waveform equalizer used in digital terrestrial broadcasting is described below, taking an example of DTV (digital television) that uses an 8 VSB (8-level vestigial sideband) modulation mode adopted in the United States.
FIG. 7 shows a DTV signal format employed in the U.S. This DTV signal format is composed of a region containing a data signal 380 of data such as video and audio, a region containing a field sync signal 370, and a region containing a segment synch signal 360. FIG. 8 shows a format of the field sync signal 370. As illustrated, the field sync signal 370 includes a PN511 signal 371, three PN63 signals 372, and a control signal 373. Note here that field sync signal #2 differs from field sync signal #1 only in that the value of the middle PN63 signal 372 is inverted. In FIG. 8, the values shown on the left side (+7, +5, +3, +1, −1, −3, −5, −7) are the eight levels used in the 8 VSB modulation mode. In this DTV signal format, one frame consists of two fields, and each field consists of 313 segments and 832 symbols.
The PN511 signal 371 is a pseudo-noise signal consisting of 511 symbols. The PN63 signals 372 are each a pseudo-noise signal consisting of 63 symbols. The control signal 373 consists of 128 symbols. Thus, the field sync signal 370 contains 828 symbols in total. Since generation methods and other details of the signals such as PN511 and PN63 are not the main features of the present invention, their explanation has been omitted here. For details on these signals, see Section 5.5.2 Data Field Sync in “ATSC Standard: Digital Television Standard (A/53), Revision C” (Advanced Television Systems Committee: www.atsc.org).
The following gives a brief description of a DTV receiver which receives a DTV signal modulated in the 8 VSB modulation mode. FIG. 9 is a simplified block diagram of the DTV receiver. A tuner 302 receives a broadcast wave carrying the DTV signal via an antenna 301, and selects a reception channel and adjusts a gain in signal level. The tuner 302 then outputs the DTV signal to a demodulator 303. The demodulator 303 demodulates the DTV signal. A decoder 304 decodes the demodulated signal according to MPEG-2 (Moving Picture Experts Group—2) or the like. A display unit 305 outputs video and audio obtained as a result of the decoding.
The demodulator 303 is explained in more detail below. The demodulator 303 includes an AD converter 311, a sync detector 314, a waveform equalizer 312, an AGC (automatic gain control) 315, an AFC (automatic frequency control) 316, and an error corrector 313.
The AD converter 311 converts the DTV signal modulated in the 8 VSB modulation mode into digital form, and outputs it to the AGC 315 and the AFC 316. The AGC 315 outputs a level adjustment signal to the tuner 302 to keep the output of the tuner 302 at a constant level. The AFC 316 converts the DTV signal to baseband, and outputs the resulting DTV signal to the sync detector 314 and the waveform equalizer 312. The sync detector 314 detects a segment sync signal 360 and a field sync signal 370 from the DTV signal, and supplies a timing signal to the waveform equalizer 312 based on the detected signals. The waveform equalizer 312 performs waveform equalization on the DTV signal in accordance with the timing signal to remove distortion, and outputs the resulting DTV signal to the error corrector 313. The error corrector 313 performs error correction on the DTV signal, and outputs the resulting DTV signal to the decoder 304. The DTV signal converted to a digital signal by the AD converter 311 contains distortion components caused by negative effects of the transmission channel. The waveform equalizer 312 serves to remove such distortion components from the DTV signal.
As described above, a waveform equalizer is used in a receiver of digital wireless communication, for removing channel distortion caused by multipath interference or the like from a received signal. FIG. 4 is a block diagram showing a general construction of such a waveform equalizer. In the drawing, a filter unit 1 is a digital filter which yields output signal y(n) by removing channel distortion from input signal x(n) using tap coefficients C0(n) to Ck−1(n) output from a tap coefficient storage unit 12. To update the tap coefficients in the waveform equalizer, a sequential update algorithm such as LMS (least mean square) or CMA (constant modulus algorithm) is employed. An error estimation unit 2 outputs e(n) which represents an error estimated to be contained in output signal y(n), using an error evaluation function specified by the algorithm. Equations 1 and 2 respectively define e(n) according to the LMS and CMA algorithms:e(n)=y(n)−ŷ(n)  (equation 1)e(n)=y(n)×(|y(n)|2−R)  (equation 2)
FIGS. 5A to 5C show a relationship between y(n) and ŷ(n), which can be observed in binary amplitude modulation. In detail, FIG. 5A shows two signal points of a received signal having no distortion, as +1 and −1. Meanwhile, if a received signal has distortion and that distortion has not been completely removed in output signal y(n), a signal point closest to y(n) is selected from the signal points shown in FIG. 5A, as ŷ(n). Which is to say, +1 is selected as ŷ(n) in the case of an output signal corresponding to received signal 1 in FIG. 5B, and −1 is selected as ŷ(n) in the case of an output signal corresponding to received signal 2 in FIG. 5C. The LMS algorithm derives e(n) from this ŷ(n).
An update amount calculation unit 10 calculates a coefficient update amount ΔCi(n) for an ith tap, according to equation 3:ΔCi(n)=μ×e(n)×x*(n−i)  (equation 3)
where x*(n−i) is a complex conjugate of x(n−i), and μ is a constant which represents a step size that determines a speed of coefficient updates. The step size can also be called a coefficient update correction quantity.
The tap coefficient storage unit 12 solves equation 4 using tap coefficient Ci(n−1) of the ith tap stored therein and coefficient update amount ΔCi(n) output from the update amount calculation unit 10:Ci(n)=Ci(n−1)−ΔCi(n)  (equation 4)
to obtain new tap coefficient Ci(n) of the ith tap.
This tap coefficient update operation is performed for all taps i (i=0 to k−1), as a result of which the tap coefficient update operation at iteration n is complete. Such an iteration is repeated to gradually carry out waveform equalization, in order to remove channel distortion from input signal x(n).
In such a sequential update algorithm, step size μ is an important factor that affects a convergence speed of waveform equalization and a residual error after convergence. In general, when step size μ is larger, the convergence is faster, but the residual error increases. On the other hand, when step size μ is smaller, the residual error decreases, but the convergence is slower.
A problem encountered by the conventional waveform equalizer is explained in detail below, with reference to FIG. 6. As mentioned earlier, step size μ is a key determinant of the convergence speed of waveform equalization and the residual error after convergence in the conventional waveform equalizer. When step size μ is small (FIG. 6A), unnecessary tap coefficients are not generated and therefore the residual error is small. However, the convergence speed is low, as a large number of iterations are needed to reach this state. When step size μ is large (FIG. 6B), the number of iterations decreases and so the convergence speed is high. However, unnecessary tap coefficients such as i=−1 and i=3 are generated, which causes an increase in residual error. This raises a demand for a waveform equalizer that achieves both a high convergence speed and a small residual error.
To solve the problem of the above conventional waveform equalizer, Japanese Patent Application Publication H11-313013 (hereafter “patent document 1”) proposes a waveform equalizer in which a tap coefficient fixing unit for fixing tap coefficients of low power taps to 0 is provided between a tap coefficient storage unit and a filter unit.
According to this technique, the tap coefficient fixing unit uniformly fixes tap coefficients which are smaller than a predetermined threshold value, to 0. This causes not only unnecessary tap coefficients but also small tap coefficients necessary for removing small multipath effects, to be changed to 0. As a result, the small multipath effects remain without being removed, and the residual error even increases.
Which is to say, when step size μ is small (FIG. 6C), the problem of slow convergence seen in FIG. 6A remains unsolved. When step size μ is large (FIG. 6D), unnecessary tap coefficients are not generated, which may contribute to a higher convergence speed and a smaller residual error. However, there is also a possibility that the residual error may increase rather than decrease, since a small tap coefficient such as i=1 that is necessary for removing multipath is fixed to 0.
Also, Japanese Patent Application Publication No. 2000-295149 (pp. 4-5, FIG. 1) (hereafter “patent document 2”) proposes a waveform equalizer which controls coefficient update amounts depending on whether the convergence speed needs to be increased, in order to solve the problem of the conventional waveform equalizer. This waveform equalizer includes a residual distortion variation monitor means for monitoring an amount of change of distortion contained in a signal after waveform equalization with respect to time. The coefficient update amounts are controlled based on the output of this residual distortion variation monitor means.
FIG. 28 shows a construction of the waveform equalizer disclosed in patent document 2. As shown in the drawing, this waveform equalizer is roughly made up of an input terminal 201, a transversal filter 202, a distortion detector 203, a coefficient update unit 204, a distortion variation detector 205, and an output terminal 206.
It should be noted here that the distortion detector 203 corresponds to the error estimation unit 2 in FIG. 4, and the coefficient update unit 204 corresponds to the tap coefficient storage unit 12 and the update amount calculation unit 10.
A received signal input at the input terminal 201 is fed to the transversal filter 202 and the coefficient update unit 204. The distortion detector 203 receives an output signal of the transversal filter 202, and detects channel distortion in the output signal. This detection of channel distortion by the distortion detector 203 can be conducted using any of the following two methods. One method calculates a difference between the output signal of the transversal filter 202 and a known signal inserted in a transmitted signal, as an amount of distortion. The other method calculates a difference between the output signal of the transversal filter 202 and one of the signed signal points of a transmitted signal, e.g. the eight levels (+7, +5, +3, +1, −1, −3, −5, −7) in 8 VSB modulation, that is closest to the output signal, as an amount of distortion. The distortion amount calculated by the distortion detector 203 is output to the coefficient update unit 204 and the distortion variation detector 205. The coefficient update unit 204 calculates-update amounts of coefficients of the transversal filter 202 from the distortion amount received from the distortion detector 203, the input signal received from the input terminal 201, and a step size, and updates the coefficients using the update amounts. This operation is repeated to remove distortion and thereby accomplish waveform equalization. In the meantime, the distortion variation detector 205 detects a temporal change in distortion amount received from the distortion detector 203, and outputs a detection result to the coefficient update unit 204. If the output of the distortion variation detector 205 is large, the coefficient update unit 204 increases the step size to accelerate waveform equalization. If the output of the distortion variation detector 205 is small, on the other hand, the coefficient update unit 204 decreases the step size to stabilize waveform equalization.
Thus, the step size in the coefficient update unit 204 is changed depending on the output of the distortion variation detector 205. By doing so, the coefficient update amounts are decreased to stabilize waveform equalization if the amount of distortion is stable without much varying with time, and increased to accelerate waveform equalization if the amount of distortion varies with time as in the case of fading and the like.
However, this waveform equalizer has the following problems. When the distortion detector 203 uses an algorithm such as CMA which detects different distortion amounts for different signed signal points even if deviations from the signed signal points are equal (see FIG. 29), the deviation variation detector 205 wrongly detects a variation in distortion amount even when a dynamic ghost, which causes the distortion amount to vary at high speed, is actually not present.
Also, noise components like AWGN (additive white Gaussian noise) remain in the output of the transversal filter 202. This causes a temporal variation in detected distortion, which may result in incorrect control of coefficient update amounts.
Furthermore, given that the step size is controlled according to the temporal variation of distortion contained in the output of waveform equalization, there is a possibility that the operation may slip into an endless loop. Which is to say, when the received signal contains dynamic multipath interference, a temporal change in distortion is detected and the step size is increased, as a result of which the temporal change in distortion diminishes. In response, the step size is decreased, which in turn initiates a temporal change in distortion.
Although a receiver for receiving an 8 VSB DTV signal has been described here for ease of explanation, the problems stated above are not limited to such, as they are commonly seen in receivers of U.S. cable digital broadcasting, wireless LAN or ADSL, and other digital wireless communication.